Semiconductor switching circuitry



1967v .1. w. MOTTO, JR 3,299,297

SEMICONDUCTOR SWITCHING CIRCUITRY Original Filed Oct. 12, 1962 WITNESSESINVENTOR Jam 7 John W. M0110, Jr.

United States Patent 3,299,297 SEMICONDUCTOR SWITCHING CIRCUITRY John W.Motto, Jr., Greensburg, Pa., assignor to Westinghouse ElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Continuationof application Ser. No. 230,170, Oct. 12,

1962. This application Dec. 17, 1965, Ser. No. 520,030

1 Claim. (Cl. 307-885) This is a continuation of application Serial No.230,170, filed October 12, 1962, now abandoned, and assigned to the sameassignee.

The present invention relates to switching circuitry, and moreparticularly to semiconductor switching circuitry rusing controlledrectifiers having a gate turn-off characteristic.

Four-layer PNPN or NPNP semiconductor switching devices, commonly calledcontrolled rectifiers, have many advantages over existing switchingdevices. Among these advantages are: high switching efficiency, highsurge current ratings, fast switching times, high power gain, pulseturn-on and completely static operation. However, a primary limitationof such a semiconductor switch exists in its-ability to turn off currentonce it has been rendered conductive. Essentially, to turn such acontrolled rectifier off, the anode current must be reducedsubstantially to zero, which may be analogized to the characteristics ofa thyratron tube. Recently developed four-layer semiconductor switches,herein called turn-off controlled rectifiers, have the importantadditional feature of being capable of turning off current by applying anegative pulse voltage to the gate electrode, besides maintaining theother inherent advantages of controlled rectifiers. Since the turn-offcharacteristic permits the switching off of large amounts of current,the turn-off controlled rectifier may provide a very useful function invarious switching circuits.

Reference is made to copending application Serial No. 143,354, filedOctober 6, 1961, now Patent No. 3,210,563 by T. C. T. New and assignedto the same assignee as the present invention, showing a semiconductorswitching device having a gate turn-01f characteristic which could beused in the present invention.

It is, therefore, an object of the present invention to provide new andimproved switching circuitry utilizing turn-off controlled rectifiers.

It is a further object of the present invention to provide new andimproved switching apparatus utilizing turnoff controlled rectifiershaving the capability of switching large quantities of current on andoff in a time controlled manner.

Broadly, the present invention provides switching circuitry in which acontrolled rectifier having a gate turnoff characteristic is renderedconductive and non-conductive in a time controlled manner to provide aswitching function across a load.

These and other objects will become more apparent when considered inview of the following specification and drawing in which:

FIG. 1 is a schematic diagram embodying the teachings of the presentinvention; and

FIG. 2 is a schematic diagram of a circuit which may be added to thecircuit of FIG. 1.

Referring to FIG. 1, an astable, free-running multivibrator circuit orDC. chopper circuit is shown utilizing a turn-off controlled rectifierTO-CR, which may be a device such as disclosed in the copendingapplication supra. With the turn-off controlled rectifier TO-CR in itsnon-conductive, turned-off state, by connecting a source of directvoltage E-|-, not shown, to the terminal T+, the capacitor C1 willcharge to a predetermined voltage 3,299,297 Patented Jan. 17, 1967 fromthe source E+ through the resistor R1, and the load resistance RL toground. A discharge circuit inplace the function of the breakdown diode.Thus, when the voltage across the capacitor C1 reaches a predeterminedvalue, the diode D1 will break down, thus providing a positive voltagepulse to the gate electrode G of therectifier TOCR through the diode D1and the resistor R3. The application of positive potential to the gateelectrode G will cause the turn-off controlled rectifier TOCR to becomeconductive and, therefore, provide a low resistance current path fromthe potential source E+ through the anode electrode A and the cathodeelectrode K of the rectifier and then through the load resistor RL toground. The on time constant, that is the time it will be required toswitch the rectifier TO-CR from its non-conducting to conducting state,will be determined by the time constant (Rl-l-RL) C1. The turnon timemay readily be controlled by adjusting the values of these components.With the rectifier TO-CR in its conducting state, the capacitor C2 willbegin to charge from the cathode electrode K of the rectifier TO-CRthrough the capacitor C2 and the resistor R2 to ground. During the timeperiod determined by the value of the resistor R2 and the capacitor C2,current will continue to be passed through the load resistance RL. Adischarge circuit including the breakdown diode D2 and the resistor R4is connected across the capacitor C2 to the gate electrode G of theturn-01f controlled rectifier TO-CR. When the potential across thecapacitor C2 reaches a predetermined value, after a time constantdetermined by C2R2, the breakdown diode D2 will go into its high 7current, low voltage state, such that a discharge path will be providedfrom the capacitor C2 through the gate electrode G, the resistor R4 andthe diode D2. This will appear as though a negative voltage is appliedto the gate electrode G, which will cause the turn-off controlledrectifier to switch to its off, non-conducting state. With the turn-offcontrolled rectifier TO-CR having supplied a negative voltage from thedischarge of the capacitor C2, current will be blocked from passing tothe load RL from the source E-|-; thus, the circuit will be placed inits original state and the cycle will start over with the capacitor C1again beginning to charge through the resistor. R1 and the loadresistance RL to ground. The circuit thus functions as an astable,free-running multivibrator with the turn-on time being controlled by thetime constant established by the resistor R1, the load resistor RL andthe capacitor C1. The turn-off time is in turn controlled by the valuesof the resistor R2 and the capacitor C2. The values of the direct sourcepotential E+ and the breakdown values of the diodes D1 and D2 areselected so that a sufiicient potential will be applied to the gateelectrode to turn in the turn-off controlled rectifier onor off as thecase may be.

To increase the current turn-off capabilities of the turnoff controlledrectifier TO-CR, the shunt circuit of FIG. 2 may be utilized so thatadditional anode current may be switched off than with just the circuitshown in FIG. 1. So if the terminals T1 and T2 of FIG. 2 are connectedto the junctions J1 and J2, respectively, of FIG. 1 the capacitor C3effectively shunts part of the anode current during turn-01f, whichpermits the gate to effectively turn off greater anode currents. Theresistor R connected in series with the capacitor C3 is utilized tolimit the discharge current from the capacitor C3 when it is desired toturn the turn-off controlled rectifier on again during the next cycle.The diode D3 is connected in parallel across the resistor R5 with itscathode toward the capacitor C3 so that the diode effectively shunts theresistor R5 during turn off of the controlled rectifier TOCR. Using theshunt circuit of FIG. 2 has the eifect of increasing the turn-on andturn-01f time constants because of the capacitive and resistive elementsadded to the circuit; however, because of the increased turn-offcapabilities in many applications, the incorporation of this circuitinto that of FIG. 1 may be advantageous.

It should also be noted that the circuit of FIG. 1 will operate inmonostable and bistable modes as well as the astable, free-running mode.The monostable mode of operation may be obtained by either disconnectingthe resistor R1, the capacitor C1, the diode D1 and the resistor R3 andthen applying a positive pulse voltage to the gate electrode G When itis desired to turn the turn-off controlled rectifier TOCR on. Therectifier TOCR will then remain conducting until the turn off timedetermined by the time constant of the elements R2 and C2 has beenreached in the manner explained above. Alternatively, the capacitor C2,the resistor R2, the diode D2 and the resistor R4 may be disconnectedfrom the circuit, and the turn-off controlled rectifier may be turnedoff by applying a negative potential pulse to the gate electrode G whenit is desired to switch the rectifier to its non-conductive state. Thus,the controlled rectifier TO-CR will remain conductive and allow acurrent to flow through the load RL until a negative pulse is applied tothe gate electrode G and then the controlled rectifier will remain offuntil the time constant of the turn-on circuitry has been completed.Also, bistable operation may be obtained by alternately pulsing withpositive and negative pulses the gate electrode G of the turn-offcontrolled rectifier TOCR without the use of the timing circuitry orwith the timing circuitry to change the pulse repetition rate of theastable multivibrator or DC. chopper circuit.

Although the present invention has been described with a certain degreeof particularity, it should be understood that the present disclosurehas been made by Way of example, and numerous changes to details ofcircuitry in the combination and arrangement of elements may be resortedto without departing from the scope and the spirit of the presentinvention.

I claim as my invention: A switching circuit operative with a source ofpotentia and a load including:

a semiconductor controlled rectifier having a gate turn 011characteristic including a gate electrode, an anode electrode and acathode electrode, with said anode and cathode electrodes beingoperatively connected between said source of potential and said load;

a first timing circuit operative to provide a turn-on signal to saidgate electrode of said rectifier after a predetermined time and rendersaid rectifier conductive to provide a conductive path to said load,

said first timing circuit including,

a first capacitor operatively connected between said anode and cathodeelectrodes and being charged during the time period when said rectifieris nonconductive, and

a first discharge circuit including first voltage responsive meansoperatively connected across said first capacitor to said gate electrodeto permit said first capacitor to discharge after a predetermined timetherethrough when a predetermined voltage is reached across said firstcapacitor to apply a turn-on signal to said gate electrode and rendersaid rectifier conductive between said anode and cathode electrodes;

a second timing circuit operative to provide a turnotf signal to saidgate electrode of said rectifier after a predetermined time and rendersaid rectifier non-conductive,

said second timing circuit including,

a second capacitor operatively connected across the load, said secondcapacitor beginning to charge in response to said rectifier becomingconductive, and

a second discharge circuit including second voltage responsive meansoperatively connected across said second capacitor to said gateelectrode to permit said second capacitor after a predetermined timefrom the time said rectifier is rendered conductive to dischargetherethrough when a predetermined voltage is reached across said secondcapacitor to apply a turn-01f signal to said gate electrode and rendersaid rectifier nonconductive between said anode and cathode electrodes;

said first capacitor beginning to be charged in response to then0n-conduction of said rectifier so that a positive turn-on signal isapplied to said gate electrode of said rectifier to render it conductiveafter a predetermined time from the time said rectifier is renderednon-conductive; and

a shunt circuit including a third capacitor operatively connectedbetween said anode and cathode electrodes of said rectifier to increasethe amount of current that can 'be turned off by said rectifier.

References Cited by the Examiner UNITED STATES PATENTS 7/1965 Wright eta1 307-88.5 9/1965 Wright 307-885

